How to understand Header HttpServletRequest

Question

Who can tell me how to understand getHeader(), and what is difference
between getHeader() and getParameter().
normally where to set this HttpHeader?

Answer 1

Thank you for your answer.
I had read documents about the http header, and now I am puzzled with
the "header field". The Http have header like session, can I add new
header like "login" ?
I get some code from some forum, I don't understand the relation
between header and login name and password. Or I just use getParameter()
instead of getHeader()?
protected void doGet(
HttpServletRequest req,HttpServletResponse resp) {
HttpSession sess = req.getSession();
String login = sess.getAttribute("login");
if (login == null) {
String newlogin = req.getHeader("login");
String newsess = req.getHeader("session");
if (newlogin != null) {
// verify login
URLConnection con = (new URL("http://signon.com/checkauth?login="+newlogin+"&session="+newsess)).connect();
if ("yes".equals(con.getHeaderField("authenticated")))
sess.setAttribute("login",newlogin);
sess.setAttribute("authsession",newsess);
else {
// send to signon.com to login
resp.sendRedirect("http://signon.com/login.jsp?from=mainsite");
else { // already logged in